The present invention relates to mechanized farming machines commonly called brush cutters and edge trimmers, intended for cutting vegetation by means of cutting lines. This invention relates more particularly to the rotary cutting heads for such machines.
These machines are generally equipped with an internal combustion engine or electric motor which rotates, at a high speed possibly of between approximately 3000 revolutions per minute and 12,000 revolutions per minute, a rotary head supporting one or more cutting lines. During rotation of the head, and owing to the effect of the centrifugal force, the cutting line or lines are deployed radially and thus sweep a certain circular region within which they exert a cutting effect on the vegetation encountered.
At the present time, two broad categories of cutting heads are known for this kind of machine. The first type of head contains a reel of cutting line, which can be gradually paid out as the line used becomes worn or breaks. The second known type of head, to which the present invention also applies, does not use a reel of line but only one or more relatively short cutting line filaments which have to be fixed individually to the head. With regard to the latter type of cutting head, reference may be made to documents GB-A-2,214,048, NL-A-8,302,111, U.S. Pat. No. 4,924,665, GB-A-1,583,521 and U.S. Pat. No. 4,513,563.
All the current cutting heads, with or without a reel of cutting line, are difficult to reload, and/or have a complicated and expensive structure, and also pose various problems.
Thus, one drawback of cutting heads with a reel of line is the phenomenon of sticking, explained below: certain powerful machines apply considerable stress to the cutting line inside their rotary head, because of the vibrations and tensions in the line which are due to the rotation of said head at a very high speed, possibly up to 12,000 revolutions per minute. This results in a considerable rise in temperature, very close to the melting point of the usual cutting lines (from 200 to 220xc2x0 C.), and this temperature rise may cause the turns of line wound on the reel housed in the rotary head to stick.
Another observed phenomenon is xe2x80x9ceyelet breakagexe2x80x9d: a conventional cutting line, overly stressed, will end up breaking at the eyelet via which the line leaves the rotary head. This is because, as the result of repeated mechanical bending forces, the line heats up at the point where it leaves the eyelet of the head. The gradual rise in temperature of the line produces, at this point, a considerable swelling which rapidly becomes a point of weakness, which initiates line fracture; thereafter the line breaks. This phenomenon also results from the fact that the eyelets where the line leaves the current cutting heads are aluminum or brass eyelets with sharp corners, which promote line breakage.
In the case of known cutting heads without a reel, the mechanical members for retaining the cutting line filaments usually have a relatively complex structure and are not very easy to use, with the risk that, should the cutting line break, a piece of line remains jammed in said members and becomes difficult to extract.
More particularly, referring to the aforementioned documents, the prior art also includes the following features and drawbacks:
documents GB-A-2,214,048 and NL-A-8,302,111: Each line filament must be folded into a xe2x80x9cUxe2x80x9d and introduced into two parallel passageways of small cross section in the head, something which appears to be difficult;
document U.S. Pat. No. 4,924,665: The line filament is simply folded into a hook, at one end, and introduced through an opening, something which is applicable for a rigid metal wire but not applicable to a flexible line filament;
document GB-A-1,583,521: According to FIGS. 4 and 6, the line filament passes xe2x80x9cdiametrallyxe2x80x9d through the cutting head, forming several sharp bends, something which makes it difficult to install and replace the line; according to FIGS. 5 and 7, each line filament has, at one end, an enlargement, thereby requiring the use of special cutting lines instead of lines of a standard type with a constant cross section;
document U.S. Pat. No. 4,513,563: The line filament is retained by means of a knot formed in this filament (see FIG. 10), something which appears to be somewhat arbitrary: thus, a knot is difficult to form in a cutting line and the knot will have a tendency to come undone by itself over time.
Reference may also be made to document DE-A-2,444,610 (FIG. 2), which relates to a mower and not to a brush cutter or an edge trimmer, and in which the line filaments are made of metal and are retained by the fact that their two parallel portions are linked together, which means that said filaments are not easily removable and replaceable. Such a device is not suitable for the retention of flexible filaments forming two free cutting portions.
The present invention aims to eliminate the drawbacks of the current cutting heads with or without a reel, by providing an improved head which overcomes the problems of sticking and of eyelet breakage, while making it easier for the head to be reloaded with cutting line and for this line to be retained, by a particularly simple, inexpensive and effective solution.
For this purpose, the subject of the present invention is essentially a rotary cutting head for brush cutters or edge trimmers, of the kind of those using relatively short cutting line filaments individually retained on the head, this head being distinguished in that it includes, between its periphery and a central recess, at least one radial passageway of cross section corresponding to twice the cross section of a cutting line filament, in such a way that the or each cutting line filament passes through the radial passageway a first time and is folded through 180xc2x0 inside the central recess in order to pass through said radial passageway a second time, the 180xc2x0 folding of the filament forming, inside the central recess in the head, a lateral swelling of the filament which ensures its retention.
Thus, the invention provides a cutting head which, being of the xe2x80x9creel-lessxe2x80x9d type, already overcomes all the problems of sticking of the turns. In addition, this head includes a retention system for flexible cutting line filaments which requires no mechanical member, while ensuring very effective immobilization of the or each line filament, withstanding the centrifugal force (the tensile force needed to separate a line filament from the head being at least 2000 N).
In this regard, the invention relies on the surprising observation that a simple folding of a line filament made of synthetic material creates in this line, at the folding point, a lateral swelling sufficient to arrest the line because the passageway through which the line passes is suitably dimensioned. Such a swelling is obtained both with a cutting line filament of round cross section and with a line filament of polygonal, for example square, cross section. It has turned out that a line filament of generally square cross section, but with concave curvilinear sides, was conducive to the formation of the swelling retaining this line filament.
The handling needed for inserting a line filament and for folding it is simple and quick, and does not require any dismantling of the head, insofar as the central recess in the latter is directed downward, and therefore toward the outside of the machine.
According to a first possibility, the 180xc2x0 folding of the or each cutting line filament, ensuring its retention on the head by creating a swelling, may be carried out a short distance from one end of this filament, the folded part being long enough to pass through the radial passageway made in the head.
According to another possibility, the 180xc2x0 folding of the or each cutting line filament, ensuring its retention on the head by creating a swelling, is carried out approximately in the middle of the length of this filament, so as to form, from this filament, two free cutting line portions, which are parallel and approximately of the same length, extending in the same plane perpendicular to the axis of the head. Thus, a single line filament, immobilized in the head, forms two parallel active portions, thereby increasing the cutting power of the line while better distributing the force, hence increasing the longevity of the line. In addition, it has been observed that mounting two line portions xe2x80x9cin parallelxe2x80x9d, in the same plane, resulted in an appreciable reduction in the sound level during operation, even when using a smooth line of ordinary (round or square) cross section.
According to another advantageous characteristic, the or each radial passageway, made in the cutting head, has at its outlet on the periphery of said head at least one rounded adjoining surface of relatively large radius of curvature, and also of relatively large developed length. This rounded shape, providing a large bearing surface for the cutting line filament leaving the head while limiting its bending, effectively and economically prevents the undesirable phenomenon of xe2x80x9ceyelet breakagexe2x80x9d, and thus contributes to the longevity of the line. Preferably, the or each radial passageway has two such rounded bearing surfaces, arranged symmetrically, giving this passageway an outlet of flared shape, so as to provide the same advantageous effect, for a head rotating in one direction or the other.
Finally, it will be noted that the head, forming the subject-matter of the invention, can be made of both metal and of synthetic material, without dimensional limitations in terms of diameter or of height, this head possibly housing any number of cutting line filaments and being able to be used on any machine of the brush-cutter or edge-trimmer type, possibly with the insertion of an adapter for mounting it.